In anti-lock brake systems, it is important to be able to detect if the control unit is properly functioning. A malfunction in the control unit, such as by degradation of a central processing unit (CPU) or clock within a microprocessor, can cause erroneous brake control signals and, hence, improper activation or deactivation of the brakes. Another possible malfunction is the degradation of the signals from a signal converter in the control unit.
To avoid this problem, many anti-lock brake systems now have control units which utilize two separate microprocessors. These two processor designs allow redundant processing of control signals along with the facility to check the accuracy of calculated results. If the two processors disagree as to the value of calculated variables or the state of proper control signals, the control unit can take appropriate steps to disable the system to avoid erroneous operation.
For example, the Bleckmann U.S. Pat. No. 4,113,321, discloses an anti-skid electronic control system having an error detecting circuit. A secondary circuit identical to the primary arithmetic circuits is provided for the anti-skid electronic control system. The outputs of the secondary and primary circuits are connected in a predetermined manner to a comparator arrangement. If an error occurs in the secondary and/or primary circuits, the comparator will detect the error and render the entire anti-skid system inoperable.
The Bleckmann et al U.S. Pat. No. 4,546,437, discloses a method and circuit for controlling a brake slip control apparatus. Valve control signals are produced in at least two independently acting, synchronously driven logical circuit units, which may be integrated circuit configurations, complete "microcontrollers" or single-chip microcomputers, and the signals' waveforms of each of the two circuit units are compared and checked for agreement externally and internally at corresponding locations within the two circuit units. Upon the occurrence of variations in the external and/or the internal signals or the signals' waveform, there will be caused, initiated or prepared a complete or partial disconnection of the brake slip control.
The Fennel U.S. Pat. No. 4,773,072, discloses a method and a circuit configuration for suppressing short term interferences in the processing of data by means of two circuit systems. The circuit systems redundantly process the data and, for the purpose of detecting errors and interferences, between the circuit systems data is exchanged and compared for agreement. If comparators detect differences, the transferred data will be taken over and the data processing will be continued with the transferred data. This takes place in several consecutive cycles. As soon as the predetermined scope or a predetermined number of cycles with differences between the compared data is exceeded, an error will be evaluated and indicated.
The Kramer et al U.S. Pat. No. 5,074,626, discloses an anti-lock brake controller for use in a vehicle anti-lock brake system. The controller includes a main microprocessor and a fail-safe microprocessor of a different type from the main microprocessor, and a device interconnecting the main and fail-safe microprocessors and the remainder of the vehicle braking system so that the fail-safe microprocessor can disable the brake release system independently of the main microprocessor.
The Gloceri U.S. Pat. No. 5,193,886, discloses a wheel speed verification system including an anti-skid control circuit and a method of processor computation confirmation within such an anti-skid control circuit. The confirmation of proper processor operation is done only at a certain one of the computation values. Rotation of a vehicle wheel provides a pulse train, the repetition rate of which is indicative of the vehicle speed. A sequence of program steps is repetitively executed in a microprocessor over a fixed time interval with the microprocessor repeating that same predetermined sequence of program steps during each successive said fixed time interval. A second pulse train is provided having one pulse for each repetition of the sequence of program steps. The speed of at least one vehicle wheel is repetitively computed according to the repetition rate of the first pulse train and the repetition rates of the first and second pulse trains are compared, and corrective action is initiated in the event that the compared repetition rates differ by more than a prescribed amount. The comparison is effected only at a certain specific wheel speed. Alternatively, a computed wheel speed may be compared to the fixed time interval and the anti-skid circuit allowed to continue in operating only so long as the compared wheel speed and time interval are within predetermined limits of one another. Again, the step of comparing is effected only at a certain specific wheel speed.
The Bleckmann et al U.S. Pat. No. 5,193,887, discloses two microcontrollers interconnected by data exchange lines for use in an anti-lock brake system. Signals are concurrently processed by the microcontrollers independently of one another and exchanged signals are checked for consistency. A deviation of the exchanged signals which is due to malfunctions is signalized to a safety circuit which, thereupon, interrupts the power supply to the solenoid valves. A monitoring signal fed to the safety circuit is a predetermined alternating signal in case of consistency of the exchanged signals and in case of proper operation of the circuit configuration. The safety circuit compares the alternating signal with a time standard derived from a clock generator which is independent of the operating cycle of the microcontrollers. A change in the alternating signals, as well as a failure in the time standard, causes a cut-off of power supply and, hence, of anti-lock control.
One of the problems with the prior art is that generally, the prior art is unable to properly handle transient signals which falsely indicate that there is a problem internal to the ABS control unit. Such transient signals may cause the anti-lock brake system to issue an erroneous brake control signal. Also, the prior art generally employs the substantially identical microprocessors which, consequently, increase the expense of the overall system.